Making zen-grid-flow-item() more functional

What do you think of my changes? They require the :nth-child pseudo selector and are therefore unsupported by IE until version 9. But with a polyfill like selectivzr this works just perfect.

As stated here I did not understand how your version of zen-grid-flow-item() could be useful:

https://github.com/JohnAlbin/compass-zen-grids/issues/33